DESCRIPTION The long-term objective of this project is to identify host defense mechanisms important in protecting the host from infection with Pneumocystis carinii. Previous work has demonstrated a critical role for the CD4+ T lymphocyte in host defense against this infection, which may explain why P. carinii pneumonia is a common complication of HIV infection. However, the precise mechanisms through which CD4+ T lymphocytes mediate host susceptibility to P. carinii infection are unknown. This application proposes to investigate the role of T lymphocyte subsets (Th1/Th2) in host responses to this infection. Preliminary data show that clearance of P. carinii infection in normal mice is temporally associated with the appearance of a cytokine profile in hilar lymph node cells consistent with Th1 lymphocytes. This project will test the experimental hypothesis that host defense against infection with P. carinii requires the early generation of specific Th1 T lymphocytes for optimal clearance of the pathogen from lung tissue. Specific Aim 1 will correlate clearance of P. carinii infection in BALB/c mice with the appearance of Th1 and Th2 lymphocytes in lung lymph nodes and parenchyma at serial intervals (3 days to 8 weeks) after inoculation. Th1/Th2 cells will be identified using quantitative RT PCR for cytokine mRNA (IFN-gamma, IL-2, IL-4, IL-5) and ELISA for cytokine production in vitro. Specific Aim 2 will confirm a protective role for Th1 lymphocytes by transfusing P. carinii-specific Th1 and Th2 lymphocytes expanded in vitro into immunocompromised scid mice prior to inoculation with P. carinii. After 4-8 weeks, the animals will be sacrificed and assayed for intensity of infection with P. carinii. Specific Aim 3 will polarize the host immune response to a Th1 or Th2 pattern in vivo by transferring murine IFN-gamma or IL-4 genes into the lungs of BALB/c mice using an adenoviral vector. The mice will then be inoculated with P. carinii organisms and assayed for clearance of infection.